Boosting the Faradaic Efficiency of Br−‐Mediated Photoelectrochemical Epoxidation by Local Acidity on α‐Fe2O3

Abstract The redox mediated photoelectrochemical (PEC) or electrochemical (EC) alkene oxidation process is a promising method to produce high value‐added epoxides. However, due to the competitive reaction of water oxidation and overoxidation of the mediator, the utilization of the electricity is far below the ideal value, where the loss of epoxidation's faradaic efficiency (FE) is ≈50%. In this study, a Br−/HOBr‐mediated method is developed to achieve a near‐quantitative selectivity and ≈100% FE of styrene oxide on α‐Fe2O3, in which low concentration of Br− as mediator and locally generated acidic micro‐environment work together to produce the higher active HOBr species. A variety of styrene derivatives are investigated with satisfied epoxidation performance. Based on the analysis of local pH‐dependent epoxidation FE and products distribution, the study further verified that HOBr serves as the true active mediator to generate the bromohydrin intermediate. It is believed that this strategy can greatly overcome the limitation of epoxidation FE to enable future industrial applications.


Figure S1
. The setup of PEC cell.The effective surface area of the α-Fe 2 O 3 photoanode was 1×2 cm 2 , and the total volume of the reaction solution in the one-compartment cell and the two-compartment cell was 5 mL and 16 mL, respectively.In the PEC epoxidation reactions, the applied bias was 0.34 V vs. Fc/Fc + and the substrate concentration was 5 mM and 1 mM in one-compartment cell and two-compartment cell respectively with 100 mM TBABF 4 and 1.3 mM Br -in the Air atmosphere at 25 °C      The PEC reactions were conducted at 0.34V vs. Fc/Fc + for photoelectrocatalysis 4.8 C.An internal standard 1,3,5-trimethoxybenzene (4, 6.12 ppm, s, 3H) was added to quantify the epoxide product (1, 3.86 ppm, t, 1H) in CD 3 CN.The conversion of 4-Br-styrene was 95%, and the corresponding selectivity and FE of epoxide product were 95% and 91%, respectively.

Figure S2 .
Figure S2.The characterizations of fresh and used α-Fe 2 O 3 photoanodes.a) XRD spectra; b) UV-vis diffuse spectra; SEM images of fresh c) and used d) α-Fe 2 O 3 ; TEM images of fresh e) and used f) α-Fe 2 O 3 .

Figure S3 .
Figure S3.The HPLC spectra of styrene epoxidation with different concentrations of Br -.As shown in the figure, the formation of 1,2-dibromo-2-phenylethane (retention time: 17.8 min) was detected on the chromatogram when the Br -concentration increased to 5 mM, and it increased with the increase of Br -concentration.

Figure S4 .
Figure S4.The HPLC spectra of styrene.The PEC reactions were conducted at 0.34 V vs.Fc/Fc + for photoelectrocatalysis 4.8 C. The substrate and epoxide product were qualified by the standard curves.The conversion of styrene was 88%, and the corresponding selectivity and FE of epoxide product were 100% and 88%, respectively.

Figure S5 .
Figure S5.The HPLC spectra of 4-F-styrene.The PEC reactions were conducted at 0.34 V vs.Fc/Fc + for photoelectrocatalysis 4.8 C. The substrate and epoxide product were qualified by the standard curves.The conversion of 4-F styrene was 89%, and the corresponding selectivity and FE of epoxide product were 95% and 82%, respectively.

Figure S6 .
Figure S6.The HPLC spectra of 4-Cl-styrene.The PEC reactions were conducted at 0.34 V vs. Fc/Fc + photoelectrocatalysis for 4.8 C. The substrate and epoxide product were qualified by the standard curves.The conversion of 4-Cl-styrene was 82%, and the corresponding selectivity and FE of epoxide product were 92% and 82%, respectively.

Figure S7 .
Figure S7.The HPLC spectra a) and the corresponding 1H NMR spectra b) of 4-Br-styrene.

Figure S14 .
Figure S14.The HPLC spectra obtained at different PEC oxidation coulomb of 1 mM styrene with 100 mM TBABF 4 and 1.3 mM Br -in a one-compartment cell at 0.34 V vs. Fc/Fc + on α-Fe 2 O 3 .

Figure
Figure S15The HPLC spectra of photoelectrolytic experiment in the two-compartment cell with 1 mM bromohydrin added in the cathodic cell.Reaction conditions: 1.3 mM Br -, 100 mM TBABF 4 , 5 M H 2 O, 1000 rpm, and 16 mL total reaction solution.

Figure S16 .
Figure S16.The photoelectrolytic epoxide results of the α-Fe 2 O 3 cycle experiments in 1 mM styrene, 5 M H 2 O and 0.1 M TBABF 4 .The nine cycles experiments of styrene epoxidation show that the conversion of styrene is maintained at ~ 95%, and the selectivity and FEs of epoxide are maintained at ~ 100% and ~ 92%, respectively.The above results indicate that the α-Fe 2 O 3 has good photoelectrochemical stability.

Figure S17 .
Figure S17.The characterizations of fresh and used BiVO 4 photoanodes.a) XRD spectra; b) UV-vis diffuse spectra; SEM images of fresh c) and used d) BiVO 4 ; TEM images of fresh e) and used f) BiVO 4 .

Figure S18 .
Figure S18.The characterizations of fresh and used TiO 2 photoanodes.a) XRD spectra; b) UV-vis diffuse spectra; SEM images of fresh c) and used d) TiO 2 ; TEM images of fresh e) and used f) TiO 2 .

Figure S19 .
Figure S19.The HPLC spectrum of styrene epoxidation with a) BiVO 4 , b) TiO 2 and c) α-Fe 2 O 3 photoanodes.The epoxidation of styrene was under the condition of Air atmosphere, 0 rpm, 1 mM styrene, 5 M H 2 O and 100 mM TBABF 4 .In the almost complete conversion of styrene substrate for BiVO 4 and TiO 2 photoanode, the selectivity of epoxy product was 30% and 4% respectively, FEs were as low as 11% and 1%, respectively.The above data show that the selectivity and FE of styrene epoxidation are increased to 99% and 88% due to the oxygen atom transfer characteristics of the α-Fe 2 O 3 photoanode.The significant 1,2-dibromo-2-phenylethane by-product is observed in the BiVO 4 and TiO 2 system (FigureS18a and b).The formation of 1,2-dibromo-2-phenylethane should step from Br 2 produced in BiVO 4 and TiO 2 system.By contrast, the selectivity for epoxide is quite high (~ 100%) and almost no dibrominated product is formed on α-Fe 2 O 3 under otherwise identical conditions.

Figure S20 .
Figure S20.The HPLC spectra of the reaction between styrene and Br 2 by adding Br 2 into the CH 3 CN solution with 5 M of water at different pHs.Br 2 molecule (0.5 M, 256 µL) was added into 10 mL H 2 SO 4 aqueous solution with pH=1, 10 mL H 2 O, and 10 mL NaOH aqueous solution with pH=13, respectively.Then the mixed solution was stirred for 20 min under ice-bath condition and the obtained solution (0.9 mL) was respectively transferred into 9.1 mL CH 3 CN with stirring under ice-bath condition to obtain 50 mM Br 2 of CH 3 CN solution with 5 M of water at different pHs.Finally, 1 mL 50 mM Br 2 of CH 3 CN solution with 5 M of water at different pHs and 1 mL 10 mM styrene solution were mixed and stirred thoroughly, and the mixed solution was tested by HPLC.As shown in Figure S19, 1,2-dibromo-2-phenylethane is formed regardless of the acidity and basicity.

Figure S21 .
Figure S21.The epoxidation performance for different concentrations (1.25 ~10 M) of H 2 O at 0.34 V vs. Fc/Fc + with 1.3 mM Br -.Each error bar denotes the standard deviation of data from three experiments.

Figure S22 .
Figure S22.The HPLC spectra of before and after reaction with 1.3 mM Br -, 1.25 M H 2 O and 16 mL total solution in the two-compartment cell.

Figure S23 .
Figure S23.The epoxidation performance of 1.25 M H 2 O, 1.3 mM Br -and 5 mM styrene system in different reaction atmospheres of a single cell.The results suggest that oxygen is not responsible for the low selectivity at 1.25 M H 2 O.

Figure S24 .
Figure S24.The IC spectra a) and the corresponding Br -concentration b) of reaction solution with electrolysis through 1.39 C before and after in the two-compartment cell.The Br -are recovered by adding trace amounts of H 2 O 2 or (and) NaOH to the reaction solution.The final recovery ratio of Br -is up to 82%, suggesting the lost Br -is present in the solution as inorganic and organic bromine.

Figure S25 .
Figure S25.Mass spectra of the styrene oxide during the oxidation of the styrene with H 2 16 O a) or H 2 18 O b) in the single cell and H 2 18 O c) in the H cell, where bromohydrin is converted to styrene oxide by adding 5 M NaOH solution.

Figure S26 .
Figure S26.The GC spectra of the headspace gas of the PEC cell after 4.8 C photoelectrolysis.The Faradaic Efficiency of the hydrogen evolution was about 95%.No dioxygen (~ 1.6 min) was detected in GC spectra, which indicates that water oxidation to O 2 rarely occurs in the low concentration Br -mediated PEC epoxidation.